The present invention regards a system and a process for efficient cooling and/or heating that is capable of using multiple sources of thermal energy, including waste heat, renewable thermal energy, and fuel combustion. The process produces cooling by mixing a solute with a solvent that produces a positive enthalpy change, and produces heating by mixing a solute with a solvent that produces a negative enthalpy change. In both cases, the solute is then separated from the solvent using thermal energy, and the cooling/heating process is repeated in a continuous fashion.
Thermally driven cooling and heating systems presently available on the market include absorption, adsorption, and ejector-compressor systems. These systems tend to be large and bulky, and require many hydraulic loops and auxiliary components, resulting in systems that require significant amounts of energy to effectively change the temperature of a space.
For example, a simple absorption refrigeration system utilizes a gas-liquid mixture which forms a solution due to the strong affinity between the two fluids; the gas-rich solution is then pumped to a high-pressure zone, where the mixture is heated by a generator. Vapors of refrigerant generated in this pressurized heating process are sent towards the traditional refrigeration cycle of a condenser, an expansion valve and an evaporator, wherein the temperature is reduced by the evaporation of refrigerant in the evaporator, at low pressures. The now gas-poor solution turns over in the absorber by passing through a pressure-relief valve, and absorbs the vapors of refrigerant, allowing the cycle to begin again. The system requires multiple auxiliary components to handle its hydraulic design, and high electrical demands to handle the pressurized portions of the system.
The adsorption system operates on the principle of physical adsorption between the refrigerant and an adsorbent (liquid or solid). The molecules of the refrigerant come to be fixed at the surface of the adsorbent via van der Waals forces. The system generally consists of a generator, a condenser, a pressure-relief valve and an evaporator. Adsorption systems are limited, however, due to the weak mass and heat transfer characteristics of the adsorbent beds. Specifically, the adsorbents, such as activated carbon, zeolite or silica gel, have low thermal conductivities and poor porosity characteristics. Consequently, the system typically has a bulky collector/generator/adsorber component which requires excessive heating capacity, leading to a rather low thermal coefficient of performance (COP).
Ejector-compressor systems have become a topic of interest for research in recent years, because they are heat-operated by low grade energy sources such as solar energy and industrial waste heat. These systems can be satisfactorily operated at generator temperatures as low as 65° C. However, similar to the absorption system, they require many hydraulic loops and high auxiliary electricity loads.
The present invention relates to a unique use of enthalpy change of solution for cooling and heating applications, using liquid binary mixtures as the working fluids (refrigerant/heater). The system has a simple hydraulic design and a low electrical demand to operate its recirculation pumps.
Using these liquid binary mixtures, when a solute is mixed in a solvent resulting in a positive enthalpy change, the mixing process is described as endothermic, while a negative enthalpy change signifies an exothermic process. In endothermic processes, the solution absorbs energy in the form of heat from the surroundings, lowering the temperature of the surrounding area. For example, instant ice packs use an endothermic reaction of ammonium nitrate (NH4NO3) in water to achieve rapid cool temperatures (ΔHsol=+25.7 kJ/mol). In exothermic processes, when the enthalpy change is negative, the solution rejects heat to the surroundings, raising the temperature of the surrounding area. For example, instant hot packs use an exothermic process of magnesium sulfate (MgSO4) mixing in water, resulting in an enthalpy change of ΔHsol=−1,278.12 kJ/mol. These two examples of endothermic and exothermic processes are one-time use devices.
The present invention takes advantage of the endothermic and/or exothermic processes of certain solute/solvent combinations to modify the temperature of a space, using a continuously operating device by repeating the process of mixing and separating the solute and the solvent, allowing the process to continue without having to replace the solute or the solvent.